1. Field of the Invention
This invention relates generally to stoving lacquers and, more particularly, to new binder mixtures containing alkyd resins and hydroxyfunctional poly-N-acyl alkyleneimines, to a process for the production of these binder mixtures and to their use.
2. Statement of Related Art
Basically, lacquers are divided into physically drying systems and chemically drying systems. With chemically drying systems, a chemical reaction leading to crosslinking in the lacquer film takes place after application of the lacquer to the surface, resulting in a drastic reduction in the redissolvability of the hardened film in relation to the original lacquer ingredients.
One form of chemically drying lacquers are oxidatively drying systems where the crosslinking reaction is initiated with atmospheric oxygen. Another form of chemical drying is the curing of binders by crosslinking with crosslinking agents. One example of this second type of drying is the crosslinking of alkyd resins (binder) with melamine/formaldehyde condensates (cross-linking agent). Crosslinking can be initiated both by stoving at high temperatures (stoving lacquers) and by addition of acids (acid-curing lacquers).
Where alkyd resins are exclusively used as binders for stoving lacquers, film quality is often in need of improvement in regard to the hardness and corrosion resistance of the lacquer films. Attempts have been made to overcome these disadvantages by modifying the alkyd resins, for example with epoxides or isocyanates. Epoxy-modified alkyd resins tend to chalk. Isocyanate-modified alkyd resins produce lacquer films which tend to yellow.
DE-A-39 43 111 describes stoving lacquers containing hydroxyfunctional poly-N-acyl alkyleneimines as binders. For film formation, the hydroxyfunctional poly-N-acyl alkyleneimines are crosslinked with polyisocyanates or aminoplastics. The films formed all show acceptable hardness values, but only when crosslinking is carried out at high stoving temperatures (beyond 160.degree. C.). However, high stoving temperatures always mean long reaction times and a considerable energy demand.
In the search for binders capable of leading to paints with hard glossy films characterized by improved resistance to corrosion, tests were carried out with mixtures of the hydroxyfunctional poly-N-acyl alkyleneimines known from DE-A-39 43 111 and alkyd resins. However, it was found that, with melamine resins as crosslinker, binder mixtures such as these lead only to hazy lacquer films. Accordingly, the mixtures in question are incompatible or only poorly compatible with melamine resins. Mixtures of the poly-N-acyl alkyleneimines known from DE 39 00 859, which do not contain any hydroxyl groups, and alkyd resins were also tested. These binder mixtures also led to hazy lacquer films with melamine resins.